JP2003075404A - Gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gas sensor by which the concentration of a gas such as the concentration of e.g. hydrogen gas or the like can be measured precisely. SOLUTION: In the hydrogen gas sensor, a first electrode 3 is installed on one face of a proton conduction layer 1, a second electrode 5 and a reference electrode 7 are installed on the other face of the layer 1 so as to face the first electrode 3, and they are supported in a support 9 composed of a first support 9a and a second support 9b. A diffusion-controlled part 19 by which an ambient atmosphere communicates with a first recess 11a (consequently, the first electrode 3) is installed at the first support 9a in the support 9. That is to say, the part 19 is a small hole which introduces a fuel gas (consequently, the contained hydrogen gas) as a gas to be measured to the side of the first electrode 3 and which controls its diffusion, and it is installed so as to be situated in a direction separated from the reference electrode 7 from the end part T on the side of the reference electrode 7 of the second electrode 5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas sensor such as a hydrogen gas sensor for measuring the hydrogen gas concentration in the fuel gas of a fuel cell.

[0002]

2. Description of the Related Art In recent years, fuel cells have been extensively researched as a highly efficient and clean power source amid environmental problems on a global scale. Among them, a polymer electrolyte fuel cell (PEFC) is expected for automobiles and households due to advantages such as low temperature operation and high power density.

In the case of this fuel cell, it is promising to use a reformed gas as the fuel gas, but in order to improve efficiency and the like, a sensor which can directly detect hydrogen in the reformed gas is required. . The above sensor has a low operating temperature (less than about 100 ° C) because it is measured in a hydrogen-rich atmosphere.
is necessary.

As such a low temperature operation type sensor, for example, in the publication of EP11103807A2, as shown in FIG. 4, a proton conductive layer P1 made of a polymer electrolyte is used, and the first electrode is formed on the surface of the proton conductive layer P1. P
2, a sensor having a structure in which the second electrode P3 and the reference electrode P4 are installed is proposed.

[0005]

However, in the above-mentioned sensor, the first electrode P2 is disposed in a portion other than the portion where the second electrode P3 is projected with the proton conducting layer P1 sandwiched, that is, the first electrode P2. Since the electrode P2 is arranged not only to face the second electrode P3 but also to the reference electrode P4 other than the second electrode P3, hydrogen (more specifically, protons that are hydrogen ions: H ⁺⁾ is transferred from the first electrode P2 to the second electrode P3. 2), there is a portion where hydrogen is difficult to be pumped (that is, a portion having a high hydrogen concentration: a portion of the mesh in FIG. 4).

Moreover, in this sensor, the first electrode P2 and the reference electrode P4 are controlled to a constant potential. In that case, the control potential is the average hydrogen concentration on the first electrode P2 and the reference. It is defined by the hydrogen concentration on the electrode P4. Therefore, if there is a portion where hydrogen is hard to be pumped like this sensor, the concentration of that portion will be high, so if you try to control the average hydrogen concentration to be low, more pumping than usual will be performed. As a result, a portion having a very low hydrogen concentration locally exists on the first electrode P2.

Further, in the sensor, as shown in FIG. 4, a part of the diffusion rate controlling portion P5 is the reference electrode P of the second electrode P3.
It exists on the right side of the end on the 4th side, but the diffusion-controlling part P5 is the second
If it exists on the right side of the end of the electrode P3 on the reference electrode P4 side,
At a portion where hydrogen is difficult to be pumped, a high concentration of hydrogen is introduced from the hydrogen-rich surrounding atmosphere side and diffuses. Therefore, if the average hydrogen concentration is controlled to be low, the A portion where the hydrogen concentration is low locally exists on the electrode P2.

As a result, in a portion where the hydrogen concentration is locally low, dissociation of methanol or the like may occur, so that the current value flowing between the first electrode P2 and the second electrode P3 becomes large, and the hydrogen gas This affects the concentration measurement and causes a problem that the hydrogen concentration cannot be measured accurately.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a gas sensor capable of accurately measuring a gas concentration such as a hydrogen gas concentration.

[0010]

Means for Solving the Problems and Effects of the Invention (1) According to the invention of claim 1, the first electrode, the second electrode and the reference electrode provided in contact with the proton conductive layer, the gas atmosphere to be measured and the first electrode A diffusion rate-controlling part provided between one electrode and
The measurement target gas (for example, hydrogen gas) in the measurement target gas introduced from the measurement target gas atmosphere side through the diffusion rate controlling part is adjusted so that the potential between the first electrode and the reference electrode becomes constant. A voltage (sufficient for obtaining a limiting current) is applied between the electrode and the second electrode to cause dissociation or decomposition or reaction, and the protons generated thereby are transferred from the first electrode to the second electrode via the proton conduction layer. The present invention relates to a gas sensor that obtains the concentration of a gas to be measured based on a limiting current generated by pumping it to an electrode. Here, in particular, a structure in which a first electrode and a second electrode are opposed to each other with a proton conductive layer interposed therebetween is used. In addition to the above, the diffusion-controlling portion is arranged in a direction away from the reference electrode from the end portion of the second electrode on the reference electrode side.

That is, according to the present invention, the diffusion rate controlling unit is
By arranging the electrode in the direction away from the reference electrode from the end on the reference electrode side, hydrogen or the like can be provided on the first electrode where the gas to be measured (for example, hydrogen) is difficult to pump (from an atmosphere such as hydrogen rich). Can be pumped before reaching, so that it is possible to prevent the presence of a locally high concentration of hydrogen or the like. As a result, when controlling the first electrode and the reference electrode to a constant potential, a locally low concentration of hydrogen or the like does not occur, and therefore dissociation of methanol or the like can be prevented. It is possible to accurately measure the concentration of the measurement target gas such as hydrogen without being affected by methanol or the like.

(2) The invention according to claim 2 is characterized in that
From the reference electrode side end of the second electrode, 1.5
The feature is that they are arranged in a direction away from each other by at least mm. That is, by making the diffusion rate controlling part 1.5 mm or more away from the reference electrode side end of the second electrode with respect to the reference electrode, it is possible to more effectively realize the local increase in hydrogen concentration. As a result, the measurement accuracy of the hydrogen gas concentration and the like is greatly improved.

(3) According to the invention of claim 3, the gas sensor is
It is characterized by being a hydrogen gas sensor that measures the hydrogen gas concentration. The present invention exemplifies that the gas sensor is a hydrogen gas sensor that measures hydrogen gas concentration.

(4) In the invention of claim 4, the gas sensor is
It is characterized by being used for measuring the hydrogen gas concentration in the fuel gas of a polymer electrolyte fuel cell. The present invention illustrates the application of the gas sensor. According to the present invention, it is possible to accurately measure the hydrogen gas concentration in the fuel gas of a polymer electrolyte fuel cell without being affected by methanol or the like.

As the structure of the gas sensor, the proton conducting layer, the first electrode, the second electrode, the reference electrode, and the diffusion rate controlling part may be supported by a support. Further, the diffusion rate controlling part is for controlling the diffusion of the measured gas (particularly the measurement target gas) introduced into the gas sensor from the measured gas atmosphere through the diffusion controlling part,
For example, it can be realized by a hole opened in the support or a porous body filled in the hole.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, an example (embodiment) of an embodiment of a gas sensor of the present invention will be described. (Example) In this example, a hydrogen gas sensor used for measuring the concentration of hydrogen gas in the fuel gas of a polymer electrolyte fuel cell will be described as an example of the gas sensor.

A) First, the structure of the hydrogen gas sensor of this embodiment will be described with reference to FIG. Note that FIG. 1 is a longitudinal sectional view of the hydrogen gas sensor. As shown in FIG. 1, in the hydrogen gas sensor of the present embodiment, the first electrode 3 is provided on one surface (the upper surface in the same figure) of the proton conducting layer 1 and the other surface of the proton conducting layer 1 (the same figure). Second electrode 5 and a reference electrode 7 are provided so as to face the first electrode 3, and they are supported in a support 9 composed of a first support 9a and a second support 9b. ing.

That is, the proton conducting layer 1 is sandwiched between the first support 9a and the second support 9b, and the first electrode 3 is covered by the first support 9a and is disposed in the first recess 11a thereof. The second electrode 5 is covered with the second support 9b and is disposed in the second recess 11b of the second support 9b.
Is covered with the second support 9b and the third recess 1
It is located in 1c.

As a method of integrally forming the hydrogen gas sensor, the proton conductive layer 1 is formed by using the first support 9a and the second support 9a.
It is possible to adopt a configuration in which the support member 9b is sandwiched by the support member 9b and to fix it with a fixing member or the like not shown, or a method of integrally fixing the support member 9b with resin or the like. The proton conducting layer 1 is capable of pumping and moving protons (H ⁺ ) from one surface side to the other surface side, for example, from the first electrode 3 side to the second electrode 5 side. The material of the proton conductive layer 1 is preferably one that operates at a relatively low temperature (for example, 150 ° C. or lower), for example, “Nafion” which is a fluororesin.
(Trademark of DuPont) can be adopted.

The first electrode 3, the second electrode 5 and the reference electrode 7
Is, for example, a porous electrode containing carbon as a main component,
A catalyst layer of Pt or the like is formed on each of the electrodes 3, 5, 7 by coating on the side in contact with the proton conductive layer 1. Further, the first electrode 3, the second electrode 5, and the reference electrode 7 are connected to the circuit via lead portions not shown, and are connected between the first electrode 3 and the second electrode 5 by a power supply (battery) 13. Voltage is applied, the voltage between the first electrode 3 and the reference electrode 7 is measured by the voltmeter 15, and the current is measured by the ammeter 17.
The current flowing between the first electrode 3 and the second electrode 5 can be measured.

Among them, the reference electrode 7 keeps the voltage between the first electrode 3 and the reference electrode 7 constant, so that when measuring the hydrogen gas concentration in the gas to be measured, disturbances such as temperature and humidity are generated. It is used to reduce the effect.
It is preferable that the reference electrode 7 be a self-generated reference electrode in order to further stabilize the hydrogen concentration at the reference electrode 7.

The support 11 is an insulator made of, for example, ceramics containing alumina as a main component, and in addition to an inorganic insulator such as ceramics, an organic insulator made of resin or the like can be adopted. Of the supports 11, the first support 9a
Is provided with a diffusion rate controlling portion 19 that communicates the ambient atmosphere with the first recess 11a (hence the first electrode 3).

That is, the diffusion rate controlling portion 19 supplies the fuel gas (therefore contained hydrogen gas) as the gas to be measured to the first electrode 3
Is a small hole (for example, diameter 0.06 mm) that is introduced into the side of the second electrode 5 and controls the diffusion thereof, and is away from the reference electrode 7 with respect to the end T of the second electrode 5 on the side of the reference electrode 7 (left in FIG. Direction). Specifically, the outer peripheral edge (on the right side in the figure) of the diffusion-controlling part is the second
The electrode 5 is arranged on the left side of the end portion T by 1.5 mm or more, for example, at a position separated from the end portion T by 8.6 mm.

By setting the inner diameter of the diffusion controlling part 19 or filling the diffusion controlling part 19 with a porous material such as alumina, the degree of controlling the diffusion can be adjusted. On the other hand, the second support 9b has a surrounding atmosphere and a second
A hole 21 having a diameter of 1.7 mm, which communicates with the recess 11b (and thus the second electrode 5), is provided.

The first support 9a and the second support 9b
Is formed by stacking and firing sheets containing ceramics, and lead portions made of, for example, Pt are formed between the sheets. That is, lead portions are provided between the sheets in the first support body 9a and the second support body 9b, and the lead portions are exposed at the bottoms of the recesses 11a to 11c, so that the electrodes 3 to 7 are formed. The electrode is in contact with the side opposite to the catalyst layer to secure conduction with the electrodes 3 to 7.

B) Next, the measurement principle and measurement procedure of the hydrogen gas sensor of this embodiment will be described. When the hydrogen gas sensor is exposed to the fuel gas, the diffusion control part
Hydrogen that has reached the first electrode 3 through the proton conductive layer 1 has a hydrogen gas concentration (specifically, hydrogen gas on both electrodes 3 and 7 side) between the first electrode 3 and the reference electrode 7. An electromotive force corresponding to the difference in concentration) is generated.

Therefore, the power source 13 controls the first electrode 3 so that the potential between the first electrode 3 and the reference electrode 7 becomes constant.
A voltage is applied between the second electrode 5 and the second electrode 5. As a result, the first
On the electrode 3, hydrogen is dissociated into protons, and the protons are pumped out to the second electrode 5 via the proton conduction layer 1 to become hydrogen again and diffuse into the atmosphere of the gas to be measured.

At that time, the value of the current flowing between the first electrode 3 and the second electrode 5 (the limit current that rises to a certain value and becomes the upper limit when the voltage is applied) is proportional to the hydrogen gas concentration. Therefore, the hydrogen gas concentration can be measured by measuring the current value. Here, the role of the reference electrode 7 will be described.

When the applied voltage is controlled by using the reference electrode 7 so that the potential between the first electrode 3 and the reference electrode 7 becomes constant, the potential between the first electrode 3 and the reference electrode 7 becomes constant. The voltage between the first electrode 3 and the second electrode 5 can be made variable so that a high voltage is applied when the hydrogen concentration is high, and a low voltage is applied when the hydrogen concentration is low. Optionally, an optimum voltage can be applied between the first electrode 3 and the second electrode 5 in a wide range of hydrogen gas concentration. This makes it possible to measure the hydrogen gas concentration in a wide range with high accuracy.

Further, even when the resistance between the first electrode 3 and the second electrode 5 changes due to the change in the temperature or H ₂ O concentration in the gas to be measured, the first electrode 3 and the second electrode 5 are not changed. Since it is possible to variably control the voltage applied between the electrode 5 and the electrode 5, even when the measurement conditions related to the hydrogen gas in the gas to be measured, H ₂ O, temperature, etc. change significantly, it is possible to achieve high accuracy. It is possible to measure the hydrogen gas concentration.

C) Next, a method for manufacturing the hydrogen gas sensor of this embodiment will be briefly described. For example, as shown in FIG. 1, the second support 9b is arranged on a base (not shown) with the second recess 11b side facing upward. Next, the proton conducting layer 1 having the first electrode 3, the second electrode 5 and the reference electrode 7 arranged on both sides is arranged thereon. That is, the second electrode 5 and the reference electrode 7 are arranged so as to be housed in the second recess 11b and the third recess 11c, respectively.

Next, the first support 9a is arranged on the proton conductive layer 1 so as to surround the first electrode 3 with the first recess 11a. In this state, that is, in the state where the proton conductive layer 1 is sandwiched between the first support 9a and the second support 9b, a fixing member or the like (not shown) is pressed in the thickness direction of the hydrogen gas sensor (vertical direction in the figure). Fix it and use it as a hydrogen gas sensor.

The side surface of the hydrogen gas sensor is covered with, for example, a resin and is hermetically sealed so that air cannot be ventilated from other than the diffusion rate-controlling portion 19. It's good if you can't ventilate
The method is not limited to this.

D) Next, an experimental example conducted to confirm the effect of this embodiment will be described. In this experiment, the measurement accuracy of the hydrogen gas concentration was investigated when the position of the diffusion control part was changed. First, within the scope of the present invention, a hydrogen gas sensor similar to that of the above-mentioned embodiment was manufactured. That is, as shown in FIG.
The reference electrode side end T of the second electrode is set as the origin (0), and the reference electrode side end KT of the outer periphery of the diffusion-controlling portion is located on the + X side (direction away from the end T) from the origin. A hydrogen gas sensor was manufactured.

As the position of the end portion KT of the diffusion rate controlling portion,
0mm (No.4), 1.5mm (No.3) from the origin,
Four samples of 6.2 mm (No. 2) and 8.6 mm (No. 1) were manufactured. On the other hand, as a sample outside the scope of the present invention,
The end portion KT of the diffusion-controlled portion was manufactured in the -X direction from the origin, specifically, -1 mm from the origin (No. 5). It should be noted that, except for the position of the diffusion-controlling portion, it was the same as the above-mentioned embodiment.

Then, the hydrogen gas concentration was measured using the hydrogen gas sensors of the above-mentioned examples and comparative examples, and the accuracy of the hydrogen gas sensor at that time, that is, the dependence of methanol was examined. The measurement conditions are as follows. In addition, here
In order to stabilize the hydrogen gas concentration, a constant minute current (for example, 10 μA) was passed from the first electrode to the reference electrode, so that the reference electrode was used as a self-generated reference electrode.

<Measurement conditions> Gas composition: H ₂ = 50%, CO ₂ = 15%, H ₂ O = 1
0%, CH ₃ OH = 0% (or 5%), N ₂ = bal. -Gas temperature: 80 ° C-Gas flow rate: 10 L / min-Set potential Vs: 150 mV (set potential between the first electrode and the reference electrode, the first electrode side being positive) Specifically, each hydrogen gas sensor When measuring the hydrogen gas concentration in the measured gas having the above gas composition, the value of the current flowing between the first electrode and the second electrode was measured.

Then, when CH ₃ OH = 0%, the current value (A
0) and the current value (A5) at the time of CH ₃ OH = 5% and the current value ratio (A0 / A5) were calculated. The measurement results are shown in FIG. In this FIG. 3, the horizontal axis represents the distance from the origin to (the end portion KT of) the diffusion rate controlling portion, and the vertical axis represents the current value ratio.

From FIG. 3, the hydrogen gas sensors of samples No. 1 to 4 of the present embodiment, that is, the end portion KT of the diffusion-controlling portion is arranged in the direction away from the end portion T of the second electrode with respect to the reference electrode. Compared with the hydrogen gas sensor of Sample No. 5 of the comparative example which is not so, the current value ratio, that is, CH ₃ OH = 0.
CH ₃ OH = 5% when the current value (sensitivity) at% is 1
It can be seen that the current value (sensitivity) ratio at that time is small and the accuracy is high.

In particular, it can be seen that the current value (sensitivity) ratio when CH ₃ OH = 5% is further reduced by setting the position at X = 1.5 mm or more. As is clear from the above, by disposing the diffusion-controlling portion in the direction away from the reference electrode from the end portion on the reference electrode side of the second electrode, dissociation of methanol can be suppressed and the hydrogen gas concentration can be accurately measured. It can be seen that the measurement can be performed.

Therefore, with the hydrogen gas sensor of this embodiment, the concentration of hydrogen gas in the fuel gas of the polymer electrolyte fuel cell can be accurately measured without being affected by methanol or the like. Needless to say, the present invention is not limited to the above-mentioned embodiments, and can be carried out in various modes without departing from the scope of the present invention.

For example, in the above-mentioned embodiment, the case where the hydrogen gas concentration in the fuel gas is measured by the hydrogen gas sensor has been taken as an example, but the gas sensor of the present invention is used when measuring the CO gas concentration in the fuel gas. Can also be applied.

[Brief description of drawings]

FIG. 1 is an explanatory view showing a hydrogen gas sensor of an embodiment in a broken manner.

FIG. 2 is an explanatory view showing a hydrogen gas sensor used for an experiment in a broken manner.

FIG. 3 is a graph showing experimental results.

FIG. 4 is an explanatory view showing a conventional hydrogen gas sensor in a broken manner.

[Explanation of symbols]

1 ... Proton conduction layer 3 ... First electrode 5 ... second electrode 7 ... Reference electrode 19 ... diffusion rate control

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Norihiko Nadanami             14-18 Takatsuji-cho, Mizuho-ku, Nagoya City, Aichi Prefecture             Inside this special ceramics company (72) Inventor Masaya Watanabe             14-18 Takatsuji-cho, Mizuho-ku, Nagoya City, Aichi Prefecture             Inside this special ceramics company (72) Inventor Noboru Ishida             14-18 Takatsuji-cho, Mizuho-ku, Nagoya City, Aichi Prefecture             Inside this special ceramics company F-term (reference) 5H026 AA06                 5H027 AA06 KK31

Claims (4)

[Claims] 1. A first device provided in contact with the proton conducting layer.
An electrode, a second electrode, and a reference electrode, and a diffusion rate controlling part provided between the measured gas atmosphere and the first electrode, and introduced from the measured gas atmosphere side through the diffusion rate controlling part. The measurement target gas in the measurement target gas is applied with a voltage between the first electrode and the second electrode so that the potential between the first electrode and the reference electrode is constant. , Dissociation or decomposition or reaction, and the protons generated thereby are pumped from the first electrode to the second electrode through the proton conductive layer, and the concentration of the gas to be measured is determined based on the limiting current. In the gas sensor to be sought, the first electrode and the second electrode have a structure facing each other with the proton conductive layer interposed therebetween, and the diffusion rate-controlling portion is provided in front of an end portion of the second electrode on the reference electrode side. The gas sensor being characterized in that disposed in a direction away from the reference electrode. 2. The diffusion controlling part is arranged in a direction away from the reference electrode by 1.5 mm or more from an end of the second electrode on the side of the reference electrode. Gas sensor. 3. The gas sensor according to claim 1, wherein the gas sensor is a hydrogen gas sensor that measures a hydrogen gas concentration. 4. The gas sensor according to claim 3, wherein the gas sensor is used for measuring a hydrogen gas concentration in a fuel gas of a polymer electrolyte fuel cell.